Add sentence or document suffix to output filename of preprocess_data.py

    encoded_docs = pool.imap(encoder.encode, fin, 25)

    #encoded_docs = map(encoder.encode, fin)

    level = "document"

    if args.split_sentences:

        level = "sentence"

    print(f"Vocab size: {tokenizer.vocab_size}")

    print(f"Output prefix: {args.output_prefix}")

    output_bin_files = {}

    output_idx_files = {}

    builders = {}

    for key in args.json_keys:

        output_bin_files[key] = "{}_{}.bin".format(args.output_prefix, key)

        output_idx_files[key] = "{}_{}.idx".format(args.output_prefix, key)

        output_bin_files[key] = "{}_{}_{}.bin".format(args.output_prefix,

                                                      key, level)

        output_idx_files[key] = "{}_{}_{}.idx".format(args.output_prefix,

                                                      key, level)

        builders[key] = indexed_dataset.make_builder(output_bin_files[key],

                                               impl=args.dataset_impl,

                                               vocab_size=tokenizer.vocab_size)